The muscles and bones of the thorax must support many critical functions, some of which may at times conflict. For example, the thorax must stiffen to maintain stability during locomotion, whereas it must expand to bring air into the lungs during inspiration. Yet, to maintain continuous locomotion, animals must breathe and locomote at the same time. An additional function that has been overlooked is reproduction. Animals that have large or many simultaneous offspring have much of the abdominal cavity occupied by offspring, which may interfere with both ventilation and locomotion. Breathing is impaired because the lungs have less room to inflate, whereas the torso may not be able to bend to accommodate locomotion, changing the basic dynamics of movement or resulting in loss of stability. Thus, how do animals carry large offspring, move and breathe at the same time? In this study, I will 1) determine the changes in locomotion (forces and kinematics) for gravid vs. non-gravid females over a range of speeds. 2) Determine the activity pattern of axial muscles during locomotion and whether the activity pattern differs when females are gravid. I will use electromyography to determine whether different muscles are recruited, and/or whether there are differences in timing or duration of activation. 3) Determine the change in ventilation (both while stationary and during locomotion) caused by the gravid condition by both air flow and anatomical means. This study will potentially reveal important design constraints in terrestrial vertebrate evolution, and may improve our understanding of fundamental physiological and biomechanical changes during human pregnancy.